Power-assisted steering

8.7.4 Power-assisted steering

Rack and pinion steering requires more turning ● Replacing the conventional hydraulic pump effort than a steering box, although this is not too

with an electric motor whilst the ram remains noticeable with smaller vehicles. However, heav-

much the same.

ier cars with larger engines or with wider tyres

A drive motor, which directly assists with which scrub more, often benefit from power

the steering and which has no hydraulic steering.

components.

Many vehicles use a belt driven hydraulic pump to supply fluid under pressure for use in

The second system uses a small electric motor the system. Inside the rack and pinion housing is

acting directly on the steering via an epicyclic

a hydraulic valve, which is operated as the pinion gear train. This completely replaces the hydraulic is turned for steering. The valve controls the flow

pump and servo cylinder. It also eliminates the of oil into a cylinder, which has a piston con-

fuel penalty of the conventional pump and greatly nected to the rack. This assists with the steering

simplifies the drive arrangements. Engine stall effort quite considerably.

when the power steering is operated at idle speed

A well designed system will retain ‘feel’ of road is also eliminated. Figure 8.17 shows an electric conditions for the driver to control the car. Steering

power steering system.

a slow moving heavier vehicle when there is little An optical torque sensor is used to measure room can be tiring, or even impossible for some

driver effort on the steering wheel. The sensor drivers. This is where power steering has its best

works by measuring light from an LED which is advantage. Many modern systems are able to make

shining through holes which are aligned in discs the power steering progressive. This means that as

at either end of a 50 mm torsion bar fitted into the the speed of the vehicle increases, the assistance

steering column.

184 Advanced automotive fault diagnosis

Figure 8.18 Camber angle force therefore has a greater effect on the front

wheels than on the rear.

Neutral steering occurs when the centre of

gravity is at the vehicle centre and the front and rear slip angles are equal. The cornering forces are therefore uniformly spread. Note, however,

Figure 8.17 Electric power steering

8.7.5 Steering characteristics

that understeer or oversteer can still occur if the cornering conditions change.

The steering characteristics of a vehicle, or in other words the way in which it reacts when cornering, can be described by one of three headings:

8.7.6 Camber

● oversteer; Note: Typical value is about 0.5° (values will ● understeer;

vary so check specs).

● neutral. On many cars, the front wheels are not mounted Oversteer occurs when the rear of the vehicle tends

vertically to the road surface. Often they are tilted to swing outward more than the front during

outwards at the top. This is called positive camber cornering. This is because the slip angle on the

(Figure 8.18) and has the following effects: rear axle is significantly greater than the front axle.

● easier steering, less turning effort required; This causes the vehicle to travel in a tighter circle,

● less wear on the steering linkages; hence the term oversteer. If the steering angle is

● less stress on main components; not reduced the vehicle will break away and all

● smaller scrub radius, which reduces the effect control will be lost. Turning the steering towards

of wheel forces on the steering. the opposite lock will reduce the front slip angle.

Understeer occurs when the front of the vehicle Negative camber has the effect of giving good tends to swing outward more than the rear during

cornering force. Some cars have rear wheels with cornering. This is because the slip angle on the rear

negative camber. With independent suspension axle is significantly smaller than the front axle.

systems, wheels can change their camber from This causes the vehicle to travel in a greater circle,

positive through neutral, to negative as the suspen- hence the term understeer. If the steering angle is

sion is compressed. This varies, however, with the not increased the vehicle will be carried out of the

design and position of the suspension hinge points. corner and all control will be lost. Turning the steering further into the bend will increase the front

8.7.7 Castor

slip angle. Front engined vehicles tend to under- steer because the centre of gravity is situated in

Note: Typical value is about 2 to 4° (values will front of the vehicle centre. The outward centrifugal

vary so check specs).

Chassis systems 185

Figure 8.19 Positive castor

The front wheels tend to straighten themselves out after cornering. This is due to a castor action.

Figure 8.20 Swivel axis

Supermarket trolley wheels automatically run straight when pushed because the axle on which

swivel axis inclination (also called kingpin inclin- they rotate is behind the swivel mounting. Vehicle

ation) is mainly for:

wheels get the same result by leaning the swivel pin mountings back so that the wheel axle is moved ● producing a self-centre action;

slightly behind the line of the swivel axis. The fur- ● improving steering control on corners; ther the axle is behind the swivel, the stronger will ● giving a lighter steering action.

be the straightening effect. The main effects of a Scrub radius, wheel camber and swivel axis positive castor angle (Figure 8.19) are:

inclination all have an effect on one another. The

swivel axis inclination mainly affects the self- self-centring action;

centring action, also known as the aligning torque. helps determine the steering torque when

Because of the axis inclination the vehicle is raised cornering.

slightly at the front as the wheels are turned. The Negative castor is used on some front wheel

weight of the vehicle therefore tries to force the drive vehicles to reduce the return forces when

wheels back into the straight-ahead position. cornering. Note that a combination of steering

geometry angles is used to achieve the desired

8.7.9 Tracking

effect. This means that in some cases the swivel axis produces the desired self-centre action so the

As a front wheel drive car drives forward the tyres castor angle may need to be negative to reduce the

pull on the road surface taking up the small amount return forces on corners.

of free play in the mountings and joints. For this reason the tracking is often set toe-out so that the wheels point straight ahead when the vehicle is

8.7.8 Swivel axis inclination

moving. Rear wheel drive tends to make the oppo- site happen because it pushes against the front

Note: Typical value is about 7 to 9° (values will wheels. The front wheels are therefore set toe-in. vary so check specs).

When the car moves, the front wheels are pushed out taking up the slack in the joints, so the wheels

The swivel axis is also known as the steering axis. again end up straight ahead. The amount of toe-in Swivel axis inclination (Figure 8.20) means the

or toe-out is very small, normally not exceeding angle compared to vertical made by the two

5 mm (the difference in the distance between the swivel joints when viewed from the front or rear.

front and rear of the front wheels). Correctly set On a strut type suspension system the angle is

tracking ensures true rolling of the wheels and broadly that made by the strut. This angle always

therefore reduced tyre wear. Figure 8.21 shows leans in towards the middle of the vehicle. The

wheels set toe-in and toe-out.

186 Advanced automotive fault diagnosis

the inner edge of

wheel turn outwards.The

the wheel

result of this is that the wheel with the greatest braking force is turned out with greater torque. Under different road conditions this can have the effect of producing an unwanted steering angle

Zero

The contact point

This makes the steering

of the steering axis

heavy when the vehicle is at

hits the road at the

rest because the wheel

same place as the

cannot roll at the steering

wheel centre

angle. However, no separate turning torque about the steering axis is created

From the information given you will realise that decisions about steering geometry are not clear- cut. One change may have a particular advantage in one area but a disadvantage in another. To assist with fault diagnosis a good understanding of steering geometry is essential.

Figure 8.21 Tracking